1. Field of the Invention
The present invention relates to the field of liquid or vapor metal-organic solutions that are used to form solid metal oxide thin films. The liquid solutions facilitate simplified processing of microelectronic devices and optical coatings because the films can be patterned by exposure to ultraviolet radiation.
2. Statement of the Problem
The production of microelectronic devices, e.g., integrated circuits, often requires patterning of circuit components, such as metal oxides and electrodes. The patterning process is a complicated and expensive one that normally requires the application of a photo resist layer, exposure of the resist through a mask to define the device pattern, etching to remove thin film materials, and removal of the remaining resist in a solvent wash. A simpler and less expensive patterning process is needed to avoid process complexity.
The patterning process can be simplified through the use of a photosensitive precursor solution. Nakao et al, Micro-Patterning of PbZr.sub.X Ti.sub.1-X O.sub.3 Thin films Prepared By Photo Sensitive Sol-Gel Solution, 32 Jpn. J. Appl. Phys. 4141-4143 (1993), teaches the boiling of a mixture including of lead acetate trihydrate with 2-methoxyethanol to dehydrate the trihydrate. Zirconium tetra n-butoxide and titanium tetra isopropoxide are added to the partially cooled solution to commence a complexation reaction. Acetyl acetone is added to the mixture to quench the complexation and stabilize the solution. Water is added to the solution to facilitate hydrolysis. The solution is distilled to obtain 10% PZT in 2-methoxyethanol, which yields high resolution micropatterns after ultraviolet irradiation and calcination in an oxygen furnace. The Nakao et al reference reports that 0.6 micron PZT dots are produced by the exposure of a sol-gel precursor to ultraviolet light. The patterned film is developed in a water bath to remove unpolymerized portions of the film.
The Nakao et al irradiation method does not overcome problems that are inherent to the use of sol-gel solutions, namely, the difficulties of long-term storage and exposure to water. The Nakao et al solution uses sol-gel solutions that rely upon the condensation polymerization of alkoxide ligands to gel the solution. A problem with this type of solution is that it must be prepared on-site immediately prior to use because the condensation reaction proceeds until the solution can no longer be used. Thus, it is difficult or impossible to prepare large quantities of these solutions at a plant specializing in these materials. The need to prepare these solutions on-site adds expense and complexity. Additionally, even on-site, variations in the degree of polymerization over time are ultimately associated with differences in PZT film thicknesses derived from these solutions.
Soyama et al, The Formation of a Fine-Patterned Ferroelectric Thin-Film From a Sol-Gel Solution Containing a Photo-Sensitive Water Generator, Proceedings Of The International Symposium On Applied Ferroelectrics (1995), reports the addition of o-nitrobenzyl alcohol to a PZT sol-gel solution. The o-nitrobenzyl alcohol undergoes a photolytic reaction to generate water upon exposure to ultraviolet light. Soyama et al apply the solution to a silicon substrate and expose it to masked ultraviolet light, which polymerizes the exposed portion of the solution. The mask prevents polymerization of selected portions of the film by blocking ultraviolet radiation. The unexposed portions of the film on the substrate do not polymerize, and are removed by rinsing in a developer solution containing 2-methoxyethanol and 2-propanol. The resultant pattern has a resolution down to at least 10 to 20 microns.
The Soyama et al article reports a significant advance in the art, but does not overcome the fundamental drawbacks of sot-gel solutions. These drawbacks include a lack of stability in the sol-gel solutions after they are made ready for use. Furthermore, the resolution or shape of the pattern improves with exposure time, but needs improvement.
Uchida et al, Characterization Of Self-Patterned SrBi.sub.2 Ta.sub.2 O.sub.9 Thin Films From Photo-sensitive Solutions, International Symposium On Integrated Ferroelectrics (1996) reports the patterning of strontium bismuth tantalate films that were prepared from a proprietary process using a photosensitive precursor solution. Photomicrographs of the resultant thin films show the formation of dot patterns, e.g., of a 1.times.1 micron size. The patterns sometimes have tapered edges that lack sharp corners and show signs of bubbling or porosity.
Sol-gel processes have been used to make thin film ferroelectrics including lead zirconium titanate ("PZT") on gallium arsenide and silicon substrates. Melnick et al, Process Optimization and Characterization of Device Worthy Sol-Gel Based PZT for Ferroelectric Memories, 109 Ferroelectrics pp. 1-24 (1990), shows that the most common choice of sol-gel precursor is a metal alkoxide of the form M(OR).sub.X, wherein M is a metal of valence X and R is an alkyl group. Metal alkoxides are typically produced by dissolving a metal chloride in an alcohol to yield a metal alkoxide and hydrochloric acid. An alternative reaction includes the addition of ammonia to the metal chloride and the alcohol to yield the metal. alkoxide and ammonium chloride. Metal alkoxides and sol-gels are most often prepared in an alcohol solvent, and unwanted byproducts are removed by distillation.
Metal alkoxide solutions are susceptible to condensation reactions that polymerize the solution. The mere addition of water leads to the partial hydrolysis: EQU M(OR).sub.X +mH.sub.2 O.fwdarw.M(OR).sub.X-m (OH).sub.m ROH;(1)
wherein M, X, and R are defined above and m is an integer less than or equal to X. If m=X, then the reaction (1) is a total hydrolysis, which is followed by either a water condensation (2) or an alcohol condensation (3): EQU 2M(OR).sub.X-m (OH).sub.m .fwdarw.(RO).sub.X-m (OH).sub.m-1 M--O--M(OR).sub.X-m (OH).sub.m-1 +H.sub.2 O; (2) EQU or EQU 2M(OR).sub.X-m (OH).sub.m .fwdarw.(RO).sub.X-m (OH).sub.m-1 M--O--M(OR).sub.X-m-1 (OH).sub.m +ROH. (3)
Sol-gel solutions are hydrolyzed to prevent cracking of the films during the drying and annealing of the precursor film. Some hydrolysis is necessary to prevent cracking. Melnick et al at page 22 reports that sol-gel solutions produced crack-free films when they were hydrolyzed with an h-factor of 1.43 or better. Acid or base may be added to facilitate hydrolysis. Hydrolysis under highly basic conditions proceeds by a nucleophilic reaction that leads to dense, highly branched, crosslinked polymers. Hydrolysis under acidic conditions proceeds under an electrophillic reaction, which produces partially hydrolyzed monomers that condense into a more linear, tightly crosslinked network.
Sol-gels typically have a very short shelf life, and must be made ready for use just prior to the time that they are actually needed. The condensation reactions of equations (1)-(3) above produce alcohol and water byproducts that, in turn, lead to additional condensation. It may only require a few hours for a sol-gel to degrade to the point where it is no longer useful. This occurs because variations in the degree to which the solution has hydrolyzed are associated with corresponding variations in the thickness and quality of the resultant metal oxide. These variations are unacceptable because the corresponding films have markedly different electrical or optical properties from one device to another.
Metal alkoxides exhibit hydrolytic reactivity, as well as some photoreactivity, both of which result in water production. Thus, extreme precautions must be taken to avoid introducing or forming any water whatsoever into a metal alkoxide solution, or else the ensuing polymerization can ruin the solution for its intended use. These precautions include a need to protect the solutions from exposure to atmospheric water vapor. As a practical matter, sol-gels must be prepared in small batches on an as-needed basis. Small scale production also leads to variations in film quality, and this circumstance hinders the widespread commercial acceptance of sol-gels.
There remains a true need for high resolution photosensitive sol-gel solutions that have a long shelf life.